This invention relates to a color display system comprising a color display tube having a channel plate electron multiplier adjacent to a luminescent screen, the screen comprising a repeating pattern of three phosphor elements for luminescing in different colors respectively, means for scanning an electron beam across an input face of the electron multiplier to provide electron input thereto, a pair of color selection electrodes associated with each channel of the multiplier and disposed between an output face of the electron multiplier and the luminescent screen, which are operable to deflect the electron beam existing from the channels of the electron multiplier and by appropriate control of which the electron beam can be directed selectively onto each of the plurality of phosphor elements, and circuit means connected to the color selection electrodes for switching the voltage applied to the pairs of color selection electrodes between three voltage levels.
An example of such a color display system is described in published British patent application GB No. 2124017A. The pair of color selection electrodes for each channel of the electron multiplier are arranged on opposite sides of the channel axis, adjacent a coaxial aperture in an apertured extractor electrode mounted on, and insulated from, the output face of the electron multiplier which draws the electrons out of the channel. The electrodes of each pair are electrically insulated from one another and by suitably addressing the electrodes, the electron beam exiting from the aperture in the extractor electrode can be deflected to one side or the other so as to impinge upon respective ones of the plurality of phosphor elements of a repeating vertical stripe pattern to display selectively first and second colors, namely red and blue. In the undeflected state, that is with no potentials or the same potentials applied to the pairs of electrodes, the electron beam is directed onto the third phosphor element of the repeating pattern to display green. As described in the aforementioned patent application, each pair of electrodes associated with each channel is formed separate so that two electrodes are provided between adjacent channel outputs in a row, one forming part of the electrode pair associated with one channel and the other forming part of the electrode pair associated with the adjacent channel, with the respective first and second electrodes of all pairs being electrically interconnected. The multiplier channels are aligned rectilinearly in columns and respective first and second electrodes of the electrode pairs associated with each column of channels are interconnected. Alternatively, however, a single, shared, electrode may be provided between adjacent channel outputs, this electrode serving as one electrode of each of the electrode pairs associated with the adjacent channels. In this case, columns of electrodes are interconnected and alternate electrodes in a row are interconnected with alternate columns of electrodes being switched oppositely and the pattern of phosphor elements of the screen arranged accordingly. In either event, the interconnected pairs of color selection electrodes exhibit a capacitance effect during operation.
This display apparatus enables therefore a color picture to be produced using a single electron beam which is scanned in raster fashion over the input face of the electron multiplier, the required line and field scan deflectors operating on the beam prior to reaching the electron multiplier.
The display apparatus may be used in order to display television pictures according to a conventional standard scanning format, for example the PAL standard of 625 lines, 2:1 interlace, 50 Hz field format where the input red, green and blue signals are derived from an RGB source such as a camera, telecine or from a PAL decoder.
There is described in British patent application No. 8524547 a preferred mode of operating this display system which involves scanning each picture line of the standard TV signal three times during the normal 64 .mu.s, line scan period. The red, green and blue video components, having been stored in suitable line memories, are read out separately in succession at three times the usual video signal rate with the switching circuit operating to switch the color selection electrodes to display in turn the three color components of each standard picture line separately and successively during the standard line period. The color selection sequence may either be continuous, i.e. of the form RGBRGB . . . or reversing, i.e. of the form RGBBGRRGB . . . This mode of operation requires, in addition to a 15 MHz video amplifier and a 46.8 kHz line scan, six line stores, (two for each color) and, because also the color selection electrodes exhibit a capacitance effect, a three-level color selection electrode voltage switching circuit capable of charging the selection electrode capacitance to the requisite voltage levels in 1 or 2 microseconds. Test so far indicate that for a 300 mm screen diagonal tube this capacitance is likely to be in the range 10 to 100 nF and the required deflection voltage for the "outer" colors in the order of 100 volts. Thus the deflection electrode capacitance must be successively charged to levels of the order of +100, 0, and -100 volts in only 1 or 2 microseconds, a process requiring charging current pulses of many amps.
In operation of the tube, the extractor electrode adjacent the final dynode of the electron multiplier, and over which the deflection electrodes are disposed, is held at, for example, 2.25 kV while the screen electrode is held at around 10 kV with respect to a cathode potential of 0 V. The color selection electrodes are in this case maintained at a mean potential of, for example, 2.5 kV with the net switching voltages of, say, +100 V, and -100 V necessary to deflect the electron beam to the respective two "outer" phosphor stripes being superimposed around this mean potential. If the color selection electrodes are correctly aligned with the phosphor screen structure then the switching (deflection) voltages can be symmetrically arranged about the mean potential. However, it is a requirement that if the color selection electrodes and phosphor screen structure are not correctly aligned it should nevertheless be possible to get correct color phosphor excitation by asymmetric disposal of these switching voltages about the mean voltage.